1. Field of the Invention
The present invention relates to a continuous vacuum carburizing furnace used for carburizing workpieces such as steel parts. More particularly, the invention relates to a continuous vacuum carburizing furnace for continuously performing a sequence of steps including: heating a workpiece; subjecting the heated workpiece to carburizing and diffusing processes under a reduced pressure; and cooling the workpiece, the furnace characterized by suppressing the production of cementite and by providing efficient and easy control of the carburized case depth or surface carbon content of the workpiece.
2. Description of the Related Art
For the purpose of increasing the strength of steel parts, such as of low-carbon steel or low-alloy steel, it has been a conventional practice to subject the steel parts to the carburizing process for diffusing and penetrating carbon thereinto.
A variety of methods are used for carburizing workpieces including the steel parts or the like. As one example of these methods, there is known one employing a continuous vacuum carburizing furnace.
According to the prior art, the workpieces such as the steel parts are carburized using the continuous vacuum carburizing furnace as follows. As generally shown in FIG. 1, each workpiece 1 accommodated in a basket or the like is introduced in turn from a load chamber 8 into a heating chamber 2 via a door member 3 disposed at an inlet of the heating chamber 2. In the heating chamber 2, the workpieces 1 are sequentially heated. Then, one of the workpieces 1 thus heated is introduced into a carburizing chamber 4 under a reduced pressure via a door member 3a. A carburizing gas comprising gaseous hydrocarbon is fed into the carburizing chamber 4 so as to supply carbon to the workpiece 1 for carrying out carburization under the reduced pressure. Subsequently, the workpiece 1 is introduced into a diffusing chamber 5 under a reduced pressure via a door member 3b. In the diffusing chamber 5, the carbon supplied to the workpiece 1 is allowed to diffuse thereinto under the reduced pressure. Thereafter, the workpiece 1 is introduced from the diffusing chamber 5 into a cooling chamber 6 via a door member 3c. In the cooling chamber 6, the temperature of the above workpiece 1 is lowered. Subsequently, the workpiece 1 is introduced from the cooling chamber 6 into a quenching chamber 7 via a door member 3d and subjected to quenching. The quenched workpiece 1 is discharged via a door member 3e disposed at an exit of the quenching chamber 6.
In a case where the workpieces 1 are carburized in this manner, it is necessary to vary the length of heat time taken by the heating chamber 2, the length of carburizing time taken by the carburizing chamber 4 or the length of diffusion time taken by the diffusing chamber 5 in order to control the carburized case depth or the surface carbon content of the workpiece 1 and also to suppress the production of cementite. In some cases, the heat time taken by the heating chamber 2, the carburizing time taken by the carburizing chamber 4 and the diffusion time taken by the diffusing chamber 5 may be varied greatly.
However, where the heat time taken by the heating chamber 2, the carburizing time taken by the carburizing chamber 4 and the diffusion time taken by the diffusing chamber 5 are varied greatly in the aforesaid continuous vacuum carburizing furnace, there is a difficulty of continuously introducing the workpieces 1 into the heating chamber 2, the carburizing chamber 4 and the diffusing chamber 5 in sequence. This leads to inability to accomplish an efficient carburization of the workpieces 1.
More recently, there has been proposed an alternative continuous vacuum carburizing furnace including load chamber, heating chamber, carburizing chamber, diffusing chamber, cooling/holding chamber and quenching chamber, as disclosed in Japanese Unexamined Patent Publication No. 2002-146512. The furnace features a plurality of carburizing chambers, each of which is designed as a carburizing/diffusing chamber functioning as both the carburizing chamber and the diffusing chamber, and at least one of which is provided with a heating function such as to function as the heating chamber.
In such a continuous vacuum carburizing furnace, the heating chamber and the individual carburizing/diffusing chambers are reduced in pressure so as to carry out the heating process, carburizing process or diffusing process under the reduced pressure. In the meantime, individual workpieces accommodated in respective baskets are sequentially introduced into the respective carburizing/diffusing chambers so as to be subjected to the carburizing and diffusing processes.
However, the heating chamber and the carburizing/diffusing chambers discretely maintained in the reduced pressure entail cost increase. On the other hand, it is an extremely cumbersome and inefficient operation to open/close a door member to the carburizing/diffusing chamber each time each workpiece in the basket is introduced into each of the carburizing/diffusing chambers in turn and then to subject the workpiece to the carburizing and diffusing processes in the respective carburizing/diffusing chambers. In addition, the furnace is increased in size.